The present invention relates to an integrated switching and transmission network comprising line groups. Each line group is connected to an associated link module containing a transmitter and a receiver. Each link module is connected to an associated link connection consisting of a first link and at least a second link in order to transfer, in time division multiplex form, digital communication- and signal information from the transmitter to a congestion free digital switching system containing identical switch modules and from the switching system to the receiver. The switching system carries out space interchanges and time interchanges in order to switch digital information which is transferred by means of the mentioned link connections between arbitrary time division multiplex channels.
In a telecommunication system similar switch modules are used to produce switches which are easy to handle and which enable the capacity of the system to be expanded during normal operation. Switch modules only enable the system to be expanded satisfactorily if a module has been initially so constructed that its properties can remain unchanged independently of the switching capacity of the switching system. Switch modules which are included in frequency division multiplex telecommunication systems and which only perform space interchanges have been well known for a long time. Such analogue switch modules form stages interconnected by means of connection link groups which, in order to expand the system, are modified by adding further modules. Switch modules, however, which are included in time division multiplex systems are more difficult to produce because normally both space interchanges and time interchanges occur to transfer information between subscriber's lines connected to the time division multiplex switch- and transmission system. Principally similar modules can only be produced if one module carries out both types of interchanges.
It is known to use time division multiplex switching principles which, for example, are indicated as "time-space-time"- or "time-space-space-time"- or "space-time-space"- principles, the indication showing in which turn information coming in to the switching system in time division multiplex form is transferred between time stages to carry out time interchanges and space stages to carry out space interchanges.
A known module switch for time division multiplex digital information, which is described in "Colloque International de Commutation Electronique, Paris 1966, pages 513-520", mainly contains similar modules each switching between a number of input links and a number of output links, the time division multiplex format, i.e. the number of time slots per frame, being different for different types of links, and the space interchanges within a module being obtained by means of an internal multiplex format having at least as many time slots as there are information channels coming to the module. Time division multiplex links arranged between the modules have the same function as the mentioned connection link groups, one group being constituted by one single link connection having a time division multiplex format which guarantees that connections are established without congestion.
In Swedish Pat. No. 379.473 a "time-time"-switching principle is described which is achieved by means of input modules which through a single common time division multiplex connection are connected to output modules Thanks to said common time division multiplex connection a space stage is avoided and compared to the first mentioned known module switch, improved handling possibilities are obtained. When a module breakdown occurs, the faulty module is replaced by a fault free module without affecting the switching capacity of the other modules. Upon an expansion of the system, further input- and output modules are connected to the mentioned common time division multiplex connection, if required during operation.
Besides the above mentioned points concerning the modules dividing the switching system, the way in which the paths through the switch are established, i.e. the control of the switch, plays an important part in the reliability and the economics of a telecommunication system. It is known for example from the above mentioned publications, to control a digital time division multiplex switch by means of a central computer, which receives from subscribers' lines or from remote concentrators control signals defining an order for establishment of a connection. The computer finds, owing to the mentioned control signals, the addresses, indices, time slots, connection link numbers and so on which, depending on the chosen switching principle, must be transferred to the time- and space stages and the modules of the switch in order to establish the instructed connection. In the oldest time division multiplex systems an extra control communication system was arranged for the connection of the computer. In modern and improved time multiplex systems, for example the digital "time-space-time"-system being described in the Swedish Pat. No. 353.996, a relief of the computer is aimed at, at the same time as the mentioned control communication system as far as possible is included in the proper time division multiplex information transferring system.
An object of the present invention is to achieve an easily handled and an easily expandable IST (Integrated Switching and Transmission)-network whose congestionfree "space-time"-switching system has identically designed switch modules which are controlled in a decentralized manner without an extra control communication system and completely without a centrally arranged control unit or computer.